usage already exits, remove exit(2)

This now exits with the same status 1.
Document exit status in the man page.

svkbd.1

@@ -105,6 +105,8 @@ The colour for keys when they are part of an overlay
 .It Ev svkbd.overlayshiftbackground
 The colour for the secondary label on keys when they are part of an overlay
 .El
+.Sh EXIT STATUS
+.Ex -std
 .Sh SEE ALSO
 .Xr XParseGeometry 3
 .Sh AUTHORS

svkbd.c

@@ -1200,7 +1200,6 @@ main(int argc, char *argv[])
 		} else {
 			fprintf(stderr, "Invalid argument: %s\n", argv[i]);
 			usage(argv[0]);
-			exit(2);
 		}
 	}